1. Field of the Invention
The invention relates to a zoom lens, and more particularly, to a zoom lens suitable for a photographic optical system used in an image pickup apparatus such as a video camera, a digital camera, a broadcasting camera, and a silver-halide film camera.
2. Description of the Related Art
Recently, there has been a demand for the photographic optical systems used in an image pickup apparatus to have a wide angle of view, a high zoom ratio, and a small size. In particular, a zoom lens capable of reducing the thickness of a camera has been demanded. A retractable zoom lens is known that enables the storage of a camera in a casing when it is not being used. This is achieved by reducing the distances between each lens unit from that when the zoom lens is in the photographing state, in order to obtain both miniaturization of the camera and a high zoom ratio. In addition, an optical-path-bending zoom lens is known in which an optical-path-bending reflective element, such as a reflection mirror or a reflection prism capable of bending or altering the optical axis of a portion of the photographic optical system by 90 degrees is arranged within the optical path of the zoom lens in order to reduce a thickness of the camera. Furthermore, as a combination of both types of zoom lens, a retractable optical-path-bending zoom lens in which the optical-bending-type is used when the camera is used, and the lens unit located on the object side of the reflective element is retractably stored in a space generated by moving the reflective element in an optical axis direction when the camera is not used is known.
Japanese Patent Application Laid-Open No. 2007-279541 discusses an optical-path-bending variable-magnification optical system including a first lens unit of a positive refractive power, a second lens unit of a negative refractive power, a reflection mirror, a third lens unit of a positive refractive power, and a fourth lens unit of a positive refractive power. In addition, a size of the camera is reduced by folding the reflection mirror and retracting the first and second lens units when the camera is not used.
U.S. Pat. No. 7,889,436 discusses a zoom lens including a first lens unit of a positive refractive power, a second lens unit of a negative refractive power, a reflection member, a third lens unit of a negative refractive power, a fourth lens unit of a positive refractive power, and a fifth lens unit of a positive or negative refractive power. In addition, the camera is thinned through the retreat of the reflection member to the image plane side along an optical axis and the retraction of the first and second lens units when the camera is not used.
Using the retractable optical-path-bending zoom lens, it is possible to facilitate a high zoom ratio and reduction of a thickness of the camera if it is applied to the camera. However, in order to obtain such advantages, it is important to appropriately set the lens configuration of the zoom lens and set an arrangement of the reflective element in the middle of an optical path or a location of the aperture stop.
For example, it is important to appropriately set a lens configuration such as the number of lens units, an arrangement of the refractive powers of each lens unit, or movement conditions of each lens unit during zooming, a location of the aperture stop when it is arranged within the optical path, and the like. If such a configuration is not appropriate, it is difficult to obtain the aforementioned advantages.
For example, in Examples 1, 2, and 4 in Japanese Patent Application Laid-Open No. 2007-279541, the arrangement is made such that a contribution of variable power of the lens unit on the object side is larger than that of the reflection member, and a movement amount of the first lens unit is set to be smaller than that of the second lens unit. In each of such examples, a total angle of view at the wide-angle end is set to 60 to 65 degrees. For example, in order to obtain a wide angle of view in this configuration, the movement amount of the second lens unit is larger than that of the first lens unit. Therefore, at the wide-angle end, the front lens is arranged far from the position of the stop. As a result, at the wide-angle end, the front lens effective diameter tends to increase. Meanwhile, in order to reduce the movement amount of the second lens unit, it is necessary to increase the refractive power of the second lens unit, and curvature of field tends to increase due to a wide angle of view.
In Example 3 of Japanese Patent Application Laid-Open No. 2007-279541, the second lens unit is stationary during zooming. In this configuration, in order to obtain a zoom lens having a wider angle of view and a higher zoom ratio, it is necessary to increase the movement amount of the first lens unit or increase the refractive power of the second lens unit. As a result, the number of retraction stages increases so that the lens barrel diameter tends to increase. In the retractable optical-path-bending zoom lens, it is very difficult to obtain a wide angle of view and reduce the front lens effective diameter.